(1) Field of the Invention
The present invention relates to a limit stop assembly for a screw and nut linear actuator. In particular, the present invention relates to an improved limit stop assembly that is adjustably screw threaded and secured on a drive screw of the actuator apparatus. The improved limit stop assembly limits the motion of a drive nut over the drive screw while preventing frictional binding of the drive nut against the stop assembly.
(2) Description of the Related Art
Screw and nut linear actuator apparatus having a limit stop of the type provided by the present invention commonly comprise a threaded drive screw, a drive nut assembly, and a limit stop assembly. The drive nut assembly is screw threaded on the drive screw and is axially translatable over the drive screw by rotating the nut assembly relative to the screw. The limit stop assembly is releasably secured at a predetermined position on the drive screw, usually at opposite ends of the drive screw.
Prior art limit stop assemblies are commonly comprised of a split collar. A transverse pin projects from an internal annular wall of the collar into the collar center bore. The split collar is secured to a drive screw by inserting the drive screw through the center bore of the collar, and tightening the collar on the drive screw. An example of this type of limit stop assembly is disclosed in the U.S. patent of Thompson, Jr. et al. U.S. Pat. No. 4,804,809.
Prior art drive nut assemblies commonly include a nut body having an axial bore through the body. A plurality of circular grooves are formed in the annular wall of the nut body bore. A cylindrical sleeve is received in the bore of the nut body and retains a plurality of balls in space relation. The balls engage in both the threads of the drive screw and the circular grooves in the nut body. Opposite ends of the sleeve are formed with pairs of shoulders that extend from the ends of the sleeve and are spaced 180.degree. apart on the sleeve ends. The pairs of shoulders also project outwardly from the nut body. A drive nut assembly of this type is also disclosed in the U.S. Pat. of Thompson, Jr. et al. No. 4,804,809.
In use, the prior art drive nut assembly is axially translated fore and aft over the drive screw by rotating the drive screw relative to the drive nut. As the drive nut assembly approaches the prior art limit stop assembly, the sleeve shoulder projecting from the nut body engages the transverse pin of the limit stop assembly at some point as the sleeve of the drive nut assembly is rotated relative to the drive screw. When the sleeve shoulder engages the transverse pin the sleeve is held stationary relative to the drive screw and the nut body is prevented from further axial movement along the drive screw and rotates freely relative to the sleeve and screw.
A disadvantage encountered in prior art screw and nut linear actuator apparatus employing a limit stop assembly of the type described is that the nut body of the drive nut assembly will at times engage against the collar of the limit stop assembly prior to the sleeve shoulder of the nut assembly engaging the stop pin of the limit stop. This is caused by failing to accurately position the pin of the prior art limit stop assembly relative to the drive screw thread. The prior art limit stop assemblies are often secured in an adjusted position on the drive screw without taking care that the stop pin is properly positioned relative to the drive screw thread. This often results in the sleeve shoulder passing by the stop pin as the nut assembly is rotated and translated toward the collar, and the nut body frictionally engaging and binding against the limit stop collar. The frictional binding between the nut body and collar often causes damage to the machine employing the linear actuator apparatus due to the inability of the drive nut assembly to freewheel on the drive screw or to separate from the limit stop assembly.
The screw and nut linear actuator apparatus of the present invention overcomes the above described disadvantages encountered with prior art apparatus by providing an improved limit stop assembly which is threaded onto the drive screw and which has a stop means, the position of which remains constant relative to a drive screw thread regardless of the position of the limit stop along the drive screw. This ensures that the drive nut assembly will engage with the stop means and cause a member of the drive nut assembly to freewheel prior to the member engaging with and binding against the limit stop assembly.